My Mesa 5i25 board has a problem with GPIO.

Which inputs pins? 

The isolated inputs TB6 + TB5 pins 1..12 will accept from 5 to 36V

Thank you for the quick reply. 

I am trying to use input pin 0 on the TB6 tab of the Mesa 7i95 board. I am supplying 5 volts on and off, but the status of the input pin (hm2_7i95.0.gpio.000.in) is not changing in the Halshow monitor window, but the LED of the mesa board of the 0 input pin is getting on and off.
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I want to add an emergency stop and feed override button.

I have asked the same question ("Adding external switch for emergency stop and feed override") in my thread posted one day before, requesting you to look at that also if possible.

How is the input wired?

The 7I95/7I95T input pairs have an input common
so if you are supplying +5V to the input, the input common
must connect to 5V common (negative)

Note the you have to have 5V to reliably activate the input
4V is marginal (the input threshold is about 3.9V)

Make sure you are using the "WATCH" tab in halshow
not "SHOW"

Also note that  the 7I95/7I95T isolated inputs are not hal GPIO
pins but rather hal Inmux pins (0..23). 
 

Thanks for the "inmux" hint, it's so true that the 7i95 board has inmux-type input and output pins. Now I have connected the right input pin, and my external emergency stop button is working fine. I have added the following hal command line in my .hal file...

net estop-out iocontrol.0.user-enable-in
net external-estop-in hm2_7i95.0.inmux.00.input-00 => iocontrol.0.emc-enable-in

Let me know if it can be further simplified.

Kindly do me another favor by giving the piece of hal command line to do the x-axis feed override control using an external analog DC power supply from 0 to 10 volts 
 

There is no analog input on the 7I95/7I95T so maybe an
encoder would make more sense for feed rate override.
You could use either one of the high speed encoders or
one of the 4 MPG encoders built into the isolated I/O.

Note the both MesaCT and the very latest pncconf
( in LinuxCNC master ) can make  configurations for the
7I95/7I95T.